1. Field of the Invention
The present invention is directed to separation of liquid droplets from gas-liquid streams in chemical processes.
2. Background Art
Many chemical processes require take-off of a gas phase from chemical processing equipment such as chemical reactors. In some cases, the nature of the various reactants, products, and by-products facilitate removal of a gas phase substantially free of liquid. However, in other processes, considerable quantities of liquid droplets may be associated with the gas phase, and in the case where the liquid droplets can later solidify, whether due strictly to a phase change or to subsequent reaction, lines and valves may be plugged and require disassembly and cleaning or replacement. Furthermore, in many cases, the liquid droplets may constitute a loss of valuable reactants, intermediate products, or end products. For example, during preparation of polyethylene terephthalate polymers, polymer and oligomer particles may carry over with ethylene glycol and water as the latter are removed from the reactor in a vapor phase.
Many types of devices for liquid removal from gas streams are known, including cyclone separators, chill plates, filters, and the like. Packed columns efficiently remove liquid droplets, for example. However, many of these methods, for instance chill plates, are energy intensive, and others such as packed columns exhibit a severe pressure drop as well as being prone to plugging. In-line filters also suffer from these drawbacks.
Inertial separators or traps make use of the fact that a flowing gas can easily make turns that droplets with large inertia cannot. The droplets that cannot turn with the gas stream because of their inertia strike or impact a target or collecting surface onto which they are deposited. A simple pipe elbow is an example of such a separator. However, such separators are generally efficient only for droplets of materials with large inertia. Since the inertia of the droplets is measured by its mass, the size and density of the droplets is important in determining the removal efficiency.
In U.S. Pat. No. 5,181,943, liquid removal is effectuated by providing a large number of plate-type baffles across the path of a liquid-gas stream, the baffles being substantially parallel but downward sloping, and alternately extending from opposite sides of the separation device, are positioned transverse to the initial direction of flow. This device creates a high surface area serpentine path, and must be quite large if pressure drop is to be low. Since in many cases the separator must be maintained at a specific operating temperature and thus requires considerable external insulation, such devices are relatively capital intensive.
U.S. Pat. No. 5,510,017 discloses a gas-liquid separator involving two sets of concentric, radially arranged vanes, which cause a swirling flow of liquid-containing gas directed therethrough. The centrifugal forces generated cause liquid droplets to impinge upon the walls of the pipe section containing the separator, from which they are removed as bulk liquid by a series of drains. This device is of rather complex construction, and is believed to be useable only when configured for horizontal flow due to the placement of liquid-trapping baffles and drains. Moreover, conversion of linear flow to a swirling flow necessarily requires energy, which is manifested as a pressure drop.
EP 0 197 060 discloses a gas liquid separator useful in gas desulfurizing, which employs a plurality of groups of obliquely mounted large surface area slats which are sprayed with a rinsing liquid to carry away droplets impinging upon the slats. Use of a rinsing liquid is undesirable in many applications.
U.S. Pat. No. 7,004,998 (the '998 patent) discloses a gas-liquid separator that is referred to as a fishbone separator because the construction of it involves a central spine from which emanates a plurality of vanes for collecting liquid droplets. The fishbone construction described in the '998 patent is limited in that it is placed in the upstream (inlet) region of an elbow where the centerline of the elbow inlet is substantially vertical. This limitation is significant in that the conduit system carrying the gas exiting polymerization reactors tend to be large and inflexible offering only a limited number of available conduit positions for placing the gas-liquid separator. In some conduit layouts the upstream (inlet) region of an elbow may not be accessible, or an elbow as such may not be present in the conduit system.
U.S. patent application Ser. No. 11/155,756 (the '756 application) discloses a liquid separator adapted to be inserted into a conduit. The gas-liquid separation enhancer of the '756 application includes downward sloping vanes and an optionally return channel. The vanes of the '756 application are arranged in a simple fishbone pattern. The vanes may have an opening along the length of the vanes, and a bottom lip to channel accumulated liquid to the conduit wall or to sloped return channels. The '756 application utilizes sloped return channel/channels to return the captured droplets to the vessel from which the gas stream originated. With a central return channel, the vanes can be attached directly to the channel with the resulting assembly having a fishbone shape. The '756 application also discloses vanes that can be attached to a central plate or spine with the resulting spine-vanes assembly again having a simple fishbone shape. With all separation devices, an important goal is to reduce the amount of carryover with the least amount of pressure drop possible.
Accordingly, there is a need for a gas-liquid separator with improved collection efficiency and lower pressure drop that returns the captured liquid to the vessel from which the liquid originated.